In recent years, as a negative electrode material for lithium secondary batteries, carbon materials such as coke and graphite have been proposed to replace conventional lithium metals because, for example, they have superior flexibility and are not likely to cause electrodeposition of mossy lithium.
A negative electrode using the aforementioned carbon material is usually prepared by, for example, a method in which carbon powder (such as graphite and coke powder) and, as required, conductive agent powder (such as acetylene black and carbon black) are dispersed in a binding agent solution to produce a slurry, which is then coated onto a current collector metal by the doctor-blade method and subsequently dried.
Thus, conventionally, as the binding agent solution, a solution in which PVDF (polyvinylidene difluoride) is dissolved into NMP (N-methyl-2-pyrrolidone) has been used.
However, although PVDF is an excellent binding agent for integrating carbon powder, since its adhesiveness (adhesion) with a current collector metal is poor, the carbon powder detaches from the current collector metal (such as a copper plate and a copper foil) by repeated charge and discharge, thereby gradually lowering the battery capacity. That is, those batteries using PVDF have a problem in that their cycle life is generally short. The same tendency is observed also in the relation between a positive electrode active substance and the binding agent.
In addition, PVDF not only allows stable adhesion of those carbon powders themselves used as a negative electrode active substance and positive electrode active substances themselves, but also stably adheres those conductive agents blended in positive and negative electrodes, as well as a conductive agent with an active substance, thereby maintaining a balance between electrons and ions within a battery. Their adhesion (attachment) and dispersibility are consequently poor; therefore, in order to attain smooth charge and discharge, it is necessary to blend a large amount of conductive agent at the expense of the battery capacity. Moreover, since the conductive agent per se is a foreign matter to the active substance, mechanical bonding between the active substance and the conductive agent, and blending a large amount of the active substance and conductive agent cause drawbacks such as corrosion and cycle deterioration.